1. Field of the Invention
This invention pertains broadly to the field of electronics. More particularly, the invention pertains to electrical transformer systems. In still greater particularity, the invention relates to an electrical transformer-rectifier system designed to minimize line harmonics.
2. Description of the Prior Art
It is well-known to those of the transformer art that undesirable harmonic line currents may be generated during a transformer-rectifier process. The rectification of AC power to DC power itself may in general produce these undesirable current harmonics. When harmonic line currents are conducted through a power generation and distribution system that has impedences at these harmonic frequencies, harmonic voltages are created. These harmonic voltages distort line voltage fed to other power system loads, causing possible malfunction of the loads.
Line harmonics may be acceptable in some circumstances but are unacceptable in others. For example, in defense applications, it is common that a myriad of electrical devices be switched on or off a load line at various times and in various combinations. These nonlinear loads may cause severe current harmonics to appear in a load line. Such harmonics may not be tolerated by electrical devices, resulting in either a shutdown of the devices or unacceptable powering of the devices. In the latter case some devices may appear to be suitably powered when in reality line power distortions have created fluctuating device operation. In either case the cost of line harmonics may be high.
Because of this the United States Military has imposed Military Standard 1399 upon its electrical designers and contractors. The MIL STD 1399 sets power supply voltage harmonics at 5% with current harmonics being 3% of the fundamental for loads of 1 KVA or more.
Approaches toward reaching these limits may be better understood by an examination of prior attempts to do so.
For a typical three-phase AC to DC transformer-rectifier system it is common that a three-phase bridge rectifier utilize a diode pair for each input phase. The rectifier circuit "picks off" the three-phase incoming current at its peaks producing a "spiked" load current rich in line harmonics and far in frequency from the original sinusoidal current present before the rectification process.
In general, these line harmonics have been removed to a degree through the use of filters. However these filters may be substantial in number and because of their size, weight and complexities their cost may be high.
To do without filters and yet provide a transformer-rectifier system that approaches low voltage and current harmonic limitations, multiphase transformer-rectifier systems have been proposed. Some of these systems utilize transformers that convert three-phase input power into output phases that are a multiple of three such as 6, 12 or more transformation phases so that a greater number of "samples" from the original line input will be drawn.
In spite of some success, multiphase transformer-rectifier systems such as these have generally been unable to meet the rigid voltage and current harmonic limitations imposed by MIL STD 1399.
This lack of success may be attributed in part to a per cycle repetition that occurs in the rectified output waveform of the multiple-of-three phase transformers. This repetition can be avoided in part by constructing a transformer of a prime number of output phases, however prior art transformer construction techniques do not readily lend themselves to the development of such prime-numbered transformer designs.
This is due in part to the relatively more complicated phasor construction process that must take place to create relatively equal prime number output phases. In those prior art transformers that convert an input waveform of three phases into an output waveform that is a multiple of three, such as 6, 12, 15 etc., complete or whole number primary and secondary windings may be readily utilized to perform the necessary phasor construction process.
In a prime number transformer, such whole number windings may lead to the construction of phasors that do not well match each other in voltage magnitude and relative phase angle.
Some attempts at providing more accurate phasor construction have included the "tapping" of windings at an appropriate location so as to produce the necessary fractional voltage magnitude desired to construct an accurate output phasor. While perhaps this has been done successfully, the complexity of this process can be envisioned.
In such a process, it is necessary to first locate the precise winding interconnect point from amongst a multitude of winding turns and then it is necessary that a connection be made that does not conduct with adjacent windings. Again, though success has apparently been achieved utilizing this technique, the need for a less complex phasor construction process exists.